1. Field of the Invention
This invention relates to heat exchanger condensers and evaporators, particularly those used in HVAC applications. In particular, the invention relates to a device and method for detecting fouling in the tubes of a shell and tube type condenser, where cooling water flows through tubes, while a refrigerant flows through the shell and is cooled and condensed by the outside surfaces of the tubes.
2. Description of the Related Art
Refrigeration systems of the type used to cool large quantities of water, or otherwise provide a large cooling capacity, typically include a heat exchanger condenser having separated passageways. One passageway of the condenser carries a refrigerant to be condensed, and another carries the heat transfer fluid to cause the condensation, usually water. As the water travels through the tubes, it absorbs heat from the refrigerant vapor, causing the refrigerant to condense to a liquid state. After exiting the condenser, the refrigerant liquid then proceeds to an expansion valve, and then to an evaporator, where the refrigerant liquid turns from a liquid to a vapor phase. After exiting the evaporator, the refrigerant proceeds to a compressor and back to the condenser, repeating the refrigeration cycle. In the evaporator, a heat transfer fluid is cooled by the evaporation of the refrigerant. This heat transfer fluid leaving the evaporator is typically routed to a cooling system for cooling the spaces to be conditioned, or it can be used for other refrigeration purposes.
A long standing problem affecting the performance of heat exchanger condensers is fouling of the condenser tubes. Fouling occurs inside condenser tubes for several reasons. In particulate fouling, particles such as dirt and rust that are contained in the cooling water or other fluid flowing inside a tube will adhere to the tube inner surface. The presence of these particles on the inner surface of the tube reduces the heat transfer across the tube wall. In precipitation fouling, which may occur simultaneously with particulate fouling, chemicals (dissolved solids) that are contained in the cooling water or other fluid will deposit on the inner surface of the tube if the concentration of the chemicals is higher than the solubility limit at the water temperature. The precipitation of the chemicals onto the warmer tube inner surface is caused by this reverse solubility of the chemicals typically found in cooling water. The presence of precipitate fouling often results in an increase in the rate of particulate fouling, because particles such as rust or dirt will more easily adhere to the inner tube surface if chemicals have already adhered to the inner tube surface. Other types of fouling such as corrosion are also known.
In any type of fouling, particulates of material adhere to the inside surface of the condenser tubes, decreasing the heat transfer rate that occurs across walls of the tubes. The fouling results in a raised tube temperature and condensing saturation temperature, with a resulting increase in the system power requirement. In a conventional HVAC system, a relatively small amount of fouling can increase the annual utility costs to a building owner by thousands of dollars. In order to remove or substantially decrease the fouling on the condenser pipes, routine maintenance, such as tube cleaning or water treatment, must be performed on the tubes. This maintenance causes interruptions in the performance of the HVAC systems, however, and can be costly and time-consuming.
For these reasons, it is preferable to perform maintenance only when it is necessary, in order to reduce the maintenance costs, while also ensuring that the tubes do not become overly fouled so as to cause significant performance loss. It is therefore desirable to have a reliable method and apparatus for detecting the amount of fouling in the condenser tubes so that timely and proper maintenance can be provided to the condenser tubes.
Typical methods for detecting the presence and amount of waterside fouling have numerous drawbacks, such as inaccurate detection of fouling. For example, in one method, waterside fouling is suspected if the indicated pressure of the condenser increases compared to the design or start-up value. However, such an increase in pressure can be due to a number of factors independent from waterside fouling, such as the presence of non-condensable gas such as air on the refrigerant side of the tubes. The presence of a non-condensable may increase the indicated pressure by adding the partial pressure of the gas to the previous pressure. In addition, the non-condensable gas may thermally blanket the tubes, thereby increasing the heat transfer resistance, resulting in raised saturation pressure. In an operating condenser, it is often difficult to independently determine the extent to which air is in fact influencing the indicated pressure. Therefore, it is difficult to tell whether changes in performance are the result of fouling in the condenser tubes alone, or whether they are a result of other factors.
Therefore there is a need for a detection system and method that will accurately, efficiently, and inexpensively detect fouling in the condenser tubes.
The object of the present invention therefore is to provide an improved system and method for detecting fouling in a heat exchanger.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention includes a shell and tube heat exchanger system for transferring heat between a refrigerant flowing over an outer surface of a tube and a heat transfer fluid flowing through the tube. The heat exchanger system includes a tube including a first end and second end, a first temperature sensor for sensing the temperature of the tube, a device for determining the temperature of the heat transfer fluid inside the tube, and a fouling detector. The fouling detector calculates the degree of fouling in the tube based on the temperature of the tube and the temperature of the heat transfer fluid in the tube at an initial time and a later time.
In a further aspect of the invention, the invention includes a system for calculating the degree of fouling in a heat exchanger. The heat exchanger has at least one longitudinal tube inside of a shell. A first heat exchange medium flows inside the tube and a second heat exchange medium flows across the outside surface of the tube. The system includes a first temperature sensor for sensing the temperature of the tube at a first axial position of the tube, a device for determining the temperature of the first heat exchange medium inside the tube at the first axial position, and a fouling detector. The fouling detector calculates the degree of fouling in the tube based on the initial temperature difference between the temperature of the tube and the temperature of the first heat exchange medium in the tube at the first axial position, and a later temperature difference between the temperature of a surface of the tube and the temperature of the first heat exchange medium in the tube at the first axial position.
In a yet further aspect of the invention, the invention includes a method of detecting fouling in a heat exchanger having a first heat exchange fluid flowing through a tube and a second heat exchange fluid flowing over an outer surface of the tube. In the method of the present invention, a first temperature of the tube is sensed at a first axial position. A first temperature of the first heat exchange fluid in the tube at a position corresponding to the first axial position is simultaneously determined. The difference between the first temperature of the tube and the first temperature of the first heat exchange fluid in the tube is then calculated. Next, a second temperature of the tube at the first axial position is sensed after operating the condenser for a period of time, and a second temperature of the first heat exchange fluid in the tube at a position corresponding to the first axial position at the second point in time is simultaneously determined. The difference between the second temperature of the tube and the second temperature of the first heat exchange fluid in the tube is the then determined. Lastly, the fouling resistance of the tube is determined as a function of the difference between the first temperature of the tube and the first temperature of the first heat exchange fluid in the tube and the difference between the second temperature of the tube and the second temperature of the first heat exchange fluid in the tube.
In another aspect of the invention, the invention includes a method of detecting fouling in a heat exchanger having a first heat exchange fluid flowing through a tube and a second heat exchange fluid flowing over an outer surface of the tube. This method includes the steps of sensing the temperature of the first heat exchange fluid flowing through the tube and the temperature of the tube at two different times, and determining the amount of fouling in the tube based on the above sensed temperatures.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.